This invention relates to a vapor fuel purge system for an internal combustion engine.
Existing vehicle emission regulations require that evaporative emissions from the vehicle fuel tank be substantially reduced from current levels. In order to accomplish this reduction in emissions, a cannister is installed on the vehicle and is filled with a desiccant material to absorb vapors which may accumulate in the vehicle fuel tank. However, to minimize the cost of the cannister and the space required for it, a purge control valve is necessary.
For example, in a prior art vapor fuel purge system, the vapor fuel evaporated from such sources as the fuel tank or a float chamber of a carburetor is absorbed by the cannister and thereafter sucked into the intake passage leading to the engine through a purge passage, together with the purge air (fresh air). The quantitative control of the vapor fuel is effected by the purge control valve set on the cannister, which is actuated by the intake vacuum level. When the vapor fuel is thus caused to be sucked into the intake passage, the air-fuel ratio of the gaseous mixture to be sucked into the engine is apt to change. The vapor fuel purge, therefore, is stopped by operating the purge control valve, during low-speed operation and idling of the vehicle, because the driving performance and properties of the exhaust gas must be taken into consideration during those periods.
During the normal speed operation, however, if the vapor fuel from the fuel tank or the float chamber of carburetor, etc. is little in volume due to such factors as the atmospheric temperature and ambient temperature around the fuel tank, greater amount of fresh air than the vapor fuel is sucked into the engine to drastically lower the air-fuel ratio. In such an occasion, the driving performance of the vehicle is degraded. Such an incovenience is invited by the prior art mechanism that the purge control valve is opened in response to the intake vacuum level alone, which allows the valve to be opened even if the vapor fuel from the fuel tank to the cannister is little.